Fix: sink.text.pretty: support unknown event class name

Fix: sink.text.pretty: support unknown event class name

An event class's name property is optional. Add support in
`sink.text.pretty` for a nonexistent event class name. The component
prints `<unknown>`, the same way it prints this for an enumeration field
value without a corresponding mapping.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I26740a254956534d0a47b3760456314466afc4d7
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1623
Tested-by: jenkins <jenkins@lttng.org>